1. Field of the Invention
The present invention relates to a yellow toner used for development of electrostatic images formed in electrophotography, electrostatic recording, electrostatic printing, or the like.
2. Discussion of the Related Art
As a yellow toner, a toner containing a dichlorobenzidine pigment such as Pigment Yellow 17 as a colorant has been used. However, in the toner, the changes in hue by continuous printing are large depending upon the kinds of the charge control agent contained. Moreover, since dichlorobenzidine is listed as one of the environmental label Blue Angel regulation in Eco-marks in Germany, there has been expected a development of a yellow toner using a colorant without containing the dichlorobenzidine pigment.
Therefore, there has been reported a toner containing Pigment Yellow 185 excellent in the heat resistance in Japanese Patent Laid-Open No. Hei 6-118715. However, in order to obtain a more excellent toner for practical purposes, the toner is required to satisfy various properties such as environmental stability or color reproducibility, fixing ability, and changes in hues.
An object of the present invention is to provide a yellow toner having excellent environmental stability, color reproducibility, and fixing ability, and being less likely to undergo changes in hue, even when subjected to continuous printing.
The above and other objects of the present invention will be apparent from the following description.
The present invention relates to a yellow toner comprising fine resin particles and an external additive being added to surfaces of the fine resin particles, wherein the fine resin particles comprise:
(a) a resin binder comprising a polyester obtained by polycondensing an alcohol component comprising a compound represented by the formula (I): 
wherein R1 is an alkylene group having 2 or 3 carbon atoms; each of x and y is a positive number, wherein a sum of x and y is 1 to 16, with a carboxylic acid component comprising a dicarboxylic acid compound and a tricarboxylic or higher polycarboxylic acid compound;
(b) a colorant comprising a compound represented by the formula (II): 
(c) a charge control agent comprising a compound selected from the group consisting of metal compounds of a salicylic acid derivative represented by the formula (III): 
wherein each of R2, R3, and R4 is independently hydrogen atom, a linear or branched alkyl group having 1 to 10 carbon atoms, or allyl group; M is zinc, zirconium, chromium, aluminum, copper, nickel, or cobalt; m is an integer of 2 or more; and n is an integer of 1 or more; and metal compounds of a benzilic acid derivative represented by the formula (IV): 
wherein Q is boron or aluminum; m is an integer of 2 or more; and n is an integer of 1 or more; and
wherein the external additive comprises silica having an average particle size of from 35 to 350 nm.
The resin binder in the present invention comprises a polyester obtained by polycondensing an alcohol component comprising a compound represented by the formula (I): 
wherein R1 is an alkylene group having 2 or 3 carbon atoms; each of x and y is a positive number, wherein a sum of x and y is 1 to 16, preferably 1.5 to 5.0, with a carboxylic acid component comprising a dicarboxylic acid compound and a tricarboxylic or higher polycarboxylic acid compound.
The compound represented by the formula (I) includes alkylene(2 to 3 carbon atoms) oxide(average moles added 1 to 16) adduct of bisphenol A such as polyoxypropylene(2.2)-2,2-bis(4-hydroxyphenyl)propane and polyoxyethylene(2.0)-2,2-bis(4-hydroxyphenyl)propane. In addition, other alcohol components include ethylene glycol, propylene glycol, glycerol, pentaerythritol, trimethylolpropane, hydrogenated bisphenol A, sorbitol, or alkylene oxide(average moles added 1 to 16) adducts thereof, of which alkylene moiety has 2 to 4 carbon atoms. These compounds may be used alone or in admixture of two or more kinds.
It is desired that the compound represented by the formula (I) is contained in an amount of 5% by mol or more, preferably 50% by mol or more, more preferably 100% by mol or more, of the alcohol component.
The dicarboxylic acid compound includes dicarboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, fumaric acid, and maleic acid; succinic acids substituted by an alkyl group having 1 to 20 carbon atoms or by an alkenyl group having 2 to 20 carbon atoms, such as dodecenylsuccinic acid and octylsuccinic acid; acid anhydrides thereof, alkyl(1 to 8 carbon atoms) esters of these acids, and the like.
The tricarboxylic or higher polycarboxylic acid compound includes trimellitic acid, pyromellitic acid, acid anhydrides thereof, alkyl(1 to 8 carbon atoms) esters of these acids, and the like.
The dicarboxylic acid compound is contained in an amount of preferably from 70 to 99.95% by mol, more preferably from 70 to 90% by mol, of the carboxylic acid component. In addition, the tricarboxylic or higher polycarboxylic acid compound is contained in an amount of preferably from 0.05 to 30% by mol, more preferably from 10 to 30% by mol, of the carboxylic acid component.
The polycondensation of the alcohol component with the carboxylic acid component is carried out, for instance, by the reaction at a temperature of from 180xc2x0 to 250xc2x0 C. in an inert gas atmosphere, using an esterification catalyst as occasion demands.
It is preferable that the polyester has an acid value of from 0.1 to 55 mg KOH/g, a hydroxyl value of from 15 to 50 mg KOH/g, a softening point of from 90xc2x0 to 140xc2x0 C., and a glass transition temperature of 50xc2x0 to 70xc2x0 C., respectively.
In the present invention, the resin binder may comprise a linear polyester obtained without using a tricarboxylic or higher polycarboxylic acid compound. It is desired that such a linear polyester is used so that the polyester obtained from the above tricarboxylic or higher polycarboxylic acid compound is contained in an amount of 30% by weight or more, preferably 50% by weight or more, more preferably 100% by weight, of the resin binder.
The colorant in the present invention comprises a compound represented by the formula (II): 
As the colorant, there can be used commercially available products such as xe2x80x9cPaliotol Yellow D1155xe2x80x9d (commercially available from BASF).
The compound represented by the formula (II) is contained in an amount of preferably from 1 to 25 parts by weight, more preferably from 2.5 to 5.0 parts by weight, based on 100 parts by weight of the resin binder.
The charge control agent in the present invention comprises a compound selected from the group consisting of metal compounds of a salicylic acid derivative represented by the formula (III): 
wherein each of R2, R3, and R4 is independently hydrogen atom, a linear or branched alkyl group having 1 to 10 carbon atoms, or allyl group; M is zinc, zirconium, chromium, aluminum, copper, nickel, or cobalt; m is an integer of 2 or more; and n is an integer of 1 or more; and
metal compounds of a benzilic acid derivative represented by the formula (IV): 
wherein Q is boron or aluminum; m is an integer of 2 or more; and n is an integer of 1 or more.
Here, the metal compound may be either a metal salt or a metal complex. By the use of the metal compound of the formula (III), the resulting yellow toner is imparted with remarkably excellent environmental stability, and by the use of the metal compound of the formula (IV), the resulting yellow toner is imparted with remarkably excellent color reproducibility.
In the formula (III), R3 is preferably hydrogen atom, and each of R2 and R4 is preferably a branched alkyl group having 3 to 10 carbon atoms, more preferably tert-butyl group.
Among zinc, zirconium, chromium, aluminum, copper, nickel, and cobalt which are represented by M, zinc, zirconium and chromium which have an excellent effect of imparting chargeability are preferable, and zinc and zirconium are more preferable.
Commercially available products which are suitably used in the present invention where R3 is hydrogen atom, each of R2 and R4 is tert-butyl group include xe2x80x9cBONTRON E-84xe2x80x9d (M: zinc; commercially available from Orient Chemical Co., Ltd.); xe2x80x9cTN-105xe2x80x9d (M: zirconium; commercially available from Hodogaya Chemical Industries); xe2x80x9cBONTRON E-81xe2x80x9d (M: chromium; commercially available from Orient Chemical Co., Ltd.), and the like.
In addition, the metal compounds of a salicylic acid derivative can be readily prepared in accordance with the method described in CLARK, J. L. and KAO, H. (1948), J. Amer. Chem. Soc. 70, 2151. For instance, the metal compound can be obtained as a zinc compound by combining and mixing 2 moles of sodium salt of salicylic acid (containing sodium salt of salicylic acid derivatives) with 1 mole of zinc chloride in a solvent, and stirring the mixture with heating. This metal compound is crystals showing a white color, so that it does not cause coloration when dispersed in the resin binder. Metal compounds other than the zinc compound can be prepared by a method similar to that described above.
The metal compound of a salicylic acid derivative, based on 100 parts by weight of the resin binder, is contained in an amount of preferably 0.5 parts by weight or more, from the viewpoint of improving the triboelectic chargeability, and the metal salt compound is contained in an amount of preferably 10 parts by weight or less, from the viewpoint of preventing detachment of the charge control agent, more preferably from 1 to 5 parts by weight.
Commercially available products of the metal salt compound of a benzilic acid derivative represented by the formula (IV) include xe2x80x9cLR147xe2x80x9d (Q: boron; commercially available from Japan Carlit); xe2x80x9cLR-297xe2x80x9d (Q: aluminum; commercially available from Japan Carlit), and the like.
The metal salt compound of a benzilic acid derivative, based on 100 parts by weight of the resin binder, is contained in an amount of 0.3 parts by weight or more, from the viewpoint of improving the charging ratio of triboelectric charging, and the content is preferably 3 parts by weight or less, from the viewpoint of preventing lowering of the level of triboelectric charges by electroconductivity of the charge control agent, more preferably from 0.5 to 2 parts by weight.
In the present invention, as a charge control agent, besides the metal compound of a salicylic acid derivative or the metal salt compound of a benzilic acid derivative, a usually employed charge control agent can be also used together therewith.
The external additive in the present invention comprises a silica. In the present invention, it is preferable that at least one silica used as an external additive is one subjected to hydrophobic treatment with an organic silicon compound having an organic group such as trimethyl group.
The silica has an average particle size of from 35 to 350 nm, preferably from 35 to 100 nm. When a silica having a relatively large particle size as specified above is externally added, the movement of the toner is regulated during transferring, whereby clear images can be formed even when transferring is carried out a plurality of times. Incidentally, in the present invention, the particle size of the silica is obtained by using a scanning electron microscope or transmission electron microscope.
The silica having an average particle size of from 35 to 350 nm is contained in an amount of preferably from 0.1 to 5 parts by weight, more preferably 0.3 to 3 parts by weight, based on 100 parts by weight of the fine resin particles.
In the present invention, a silica having a small particle size with an average particle size of less than 35 nm, preferably 8 nm or more and less than 35 nm, more preferably from 10 to 18 nm may be fierier externally added to surfaces of the fine resin particles.
In the present invention, a commercially available product used as a silica having a large particle size includes xe2x80x9cNAX-50xe2x80x9d (commercially available from Nippon Aerosil; hydrophobic silica; average particle size: 35 nm), and the like, and commercially available products used as a silica having a small particle size include xe2x80x9cR-972xe2x80x9d (commercially available from Nippon Aerosil; hydrophobic silica; average particle size: 16 mn); xe2x80x9cR-974xe2x80x9d (commercially available from Nippon Aerosil; hydrophobic silica; average particle size: 12 nm), and the like.
The toner of the present invention is not particularly limited, and includes pulverized toners, polymerization toners, encapsulated toners, and the like. For instance, the toner can be prepared by adding an external additive to fine resin particles obtained by known methods such as kneading and pulverization method, spray-drying method, and polymerization method. As a general method, for instance, a resin binder, a colorant, a charge control agent, and the like are homogeneously blended in a mixer such as a ball-mill, and thereafter the mixture is melt-kneaded by a seal-type kneader or a single-screw or twin-screw extruder, or the like, and the kneaded mixture is cooled, pulverized, and classified, to give fine resin particles. Thereafter, the resulting fine resin particles are mixed with an external additive with stirring by using a high-speed agitating device such as Super Mixer and Henschel mixer, thereby allowing to deposit the external additive on the surface of the fine resin particles, to give a toner of the present invention. It is preferable that the toner of the present invention has a weight-average particle size of from 3 to 10 xcexcm.
Incidentally, in the toner of the present invention, there may be added in appropriate amounts additives such as a releasing agent, an electroconductive controlling agent, reinforcing fillers such as an extender and fibrous substances, an antioxidant, an anti-aging agent, and a cleaning ability improver.
The yellow toner of the present invention can be used as a nonmagnetic one-component developer when a fine magnetite particulate is not contained, or as a two-component developer by mixing the toner with a carrier. From the viewpoint of easy control of the triboelectric chargeability, it is preferably used in a nonmagnetic one-component developing device as a nonmagnetic one-component developer.